1. Field of the Invention
The present invention relates to a lighting unit suitable for illuminating a liquid crystal panel. More particularly the present invention relates to a light guide utilized in such a lighting unit.
2. Description of the Related Art
An example of a conventional liquid crystal display of front-lighting type is shown in FIGS. 14A and 14B. The illustrated liquid crystal display includes a point light source 7, a liquid crystal panel 8 and a transparent light guide 9 arranged in front of the liquid crystal panel 8. The liquid crystal display is of reflection type, provided with a reflector 80 at the back of the panel 8.
The light guide 9 has a nonflat front surface 90a and a flat rear surface 90b. The front surface 90a is formed with a plurality of projections 91. Each projection 91 includes two inclined surfaces 91a and 91b extending at different inclination angles. The rear surface 90b faces the liquid crystal panel 8. The light guide also includes firstxcx9cthird side surfaces 90cxcx9c90e extending between the front surface 90a and the rear surface 90b. The first side surface 90c is provided with a light incidence surface 92 facing the light source 7. The second side surface 90d is formed with a plurality of V-shaped recesses 93, and each recess i3 has paired surfaces 93a. The third side surface 90e is flat and disposed opposite to the second side surface 90d. 
With the above-described arrangement, light emitted from the light source 7 enters into the light guide 9 via the light incidence surface 92. Then, part of the light travels toward the second side surface 90d to be reflected on the wall surface 93a of the recess 93, and directed to the third side surface 90e. Then, the light is partially reflected toward the liquid crystal panel 8 by the inclined surface 91a of the recess 91, and emitted outside via the rear surface 90b of the light guide 9. The liquid crystal panel 8 is illuminated with the emitted light.
In the conventional liquid crystal display, the light guide 9 is used in the hope of achieving uniform illumination of the LC display 8 with the light originating from the point light source 7. However, since the recesses 93 of the conventional light guide 9 are identical in shape and size, the following problems arise.
Referring to FIG. 14B, with the conventional arrangement, the respective recesses 93 receive different amounts of light from the light source 7 due to the different distance from the light source. More specifically, a recess positioned relatively close to the light source 7 receives more light than a recess positioned relatively far. In addition, since all the recesses 93 are the same in shape and size, as described above, the wall surfaces 93a of the respective recesses 93 will reflect different amounts of light toward the side surface 90e. Thus, the light is emitted from the rear surface 90b of the light guide 9 with uneven intensity. Such nonuniform light is not preferable for illuminating the LC panel 8 since the quality of an image produced by the panel 8 deteriorates.
Further, as shown in FIG. 14B, in the conventional liquid crystal display, the light beams reflected by the wall surfaces 93a of the recesses 93 tend to be nonparallel to each other. This is because the irradiation angles of the light beams differ depending on whether a recess is located close to or apart from the light source 7. Such nonparallel beams are also responsible for the uneven light emission from the light guide 9.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a lighting device designed to uniformly illuminate an object such as a LC panel with the use of a point light source.
According to a first aspect of the present invention, there is provided a lighting unit comprising: a light source; and a light guide that includes a first side surface extending in a direction x and having a light incidence surface facing the light source, a second side surface extending in a direction y perpendicular to the direction x, a third side surface spaced from the second side surface in the direction x, and an outlet surface for exit of light. The second side surface is formed with a plurality of recesses each including an inclined surface for causing light emitted from the light source to be reflected toward the third side surface, the recesses increasing progressively in depth as they are positioned farther from the light source.
Preferably, the inclined surface of each recess forms an acute angle relative to the direction y, the angle increasing progressively as the recess is positioned farther from the light source.
Preferably, the light incidence surface inclines at an acute angle to the second side surface.
Preferably, the light source comprises a light-emitting diode chip including a light-emitting surface and inclining to the second side surface so that the light emitting surface is disposed in parallel to the light incidence surface.
According to a second aspect of the present invention, there is provided a liquid crystal display comprising: a liquid crystal panel; a light guide facing the liquid crystal panel; and a point light source arranged adjacent to the light guide. The light guide has a side surface formed with a plurality of wedge-shaped recesses that reflect light emitted from the light source, the recesses increasing progressively in depth as they are positioned farther from the light source.
According to a third aspect of the present invention, there is provided a light guide comprising: a front surface and a rear surface; a first side surface which extends in a direction x and includes an light incidence surface; a second side surface which extends in a direction y perpendicular to the direction x; a light outlet region for exit of light traveling in the direction x through either one of the front surface and the rear surface; and a light guiding region leading light cast from the light incidence surface to the light outlet region by reflection on the second side surface. The light guiding region is formed with an elongated slit extending in the direction y.
Preferably, the slit has a pair of mirror surfaces spaced apart from each other in the direction x.
Preferably, the light guiding region has a front surface and a rear surface which are covered with a light reflector.
Preferably, the light guiding region has a front surface and a rear surface which are formed with a plurality of wedge-shaped grooves extending in the direction y for reflecting light emitted from the light source.
Preferably, the second side surface is formed with a plurality of recesses each including a light reflecting surface inclining relative to the direction y.
According to a fourth aspect of the present invention, there is provided a lighting unit comprising a light guide, a light source and a light reflector. The light guide includes a front surface, a rear surface, and elongated first and second side surfaces spaced from each other. The light source emits light into the light guide so that the supplied light travels in the light guide from the first side surface toward the second side surface. The light reflector reflects light traveling from the first side surface to the second side surface within the light guide. The light guide has a surface formed with a plurality of inclined surfaces for causing the light reflected by the light reflector and traveling from the second side surface toward the first side surface to be reflected for exit through the rear surface of the light guide.
Preferably, the light reflector covers the second side surface.
Preferably, the second side surface is formed with a plurality of recesses each forming a wall surface inclining relative to a longitudinal direction of the second side surface.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.